CN110160989B - Trace gas detection method and detection device - Google Patents
Trace gas detection method and detection device Download PDFInfo
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Abstract
The application provides a trace gas detection method and a trace gas detection system, wherein the trace gas detection method fills gas to be detected in a resonant cavity, and utilizes the resonant cavity to enhance the laser power of detection light entering the resonant cavity on one hand and improve the saturation parameter of molecular transition of the trace gas to be detected; on the other hand, the effective absorption range of the trace gas to be detected is enhanced, so that the detection sensitivity of weak absorption of the trace gas to be detected is improved, the molecular saturation absorption spectrum of the trace gas to be detected is obtained, and the purpose of detecting the concentration of the trace gas can be realized by utilizing the detection light provided by a conventional laser under the normal temperature condition. And the molecular saturation absorption spectrum of the to-be-detected trace gas obtained by the trace gas detection method is effectively separated from the absorption spectrum of background gas molecules in the to-be-detected gas, so that the interference of the background gas is eliminated, and the detection of the to-be-detected trace gas is realized.
Description
Technical Field
The present disclosure relates to the field of optical detection technologies, and more particularly, to a method and an apparatus for detecting trace gas.
Background
The Molecular Absorption spectroscopy (Molecular Absorption spectroscopy) technology is a technology for measuring the concentration of target gas molecules to be measured, and specifically comprises the following steps: measuring an absorption line of a certain target gas molecule component in the gas to be measured to obtain the absorption rate of the target gas molecule, and obtaining the concentration of the target gas to be measured in the gas to be measured by utilizing the corresponding relation between the absorption rate and the concentration of the target gas molecule to be measured.
The currently common molecular absorption spectrum technology is to measure the absorption rate of a certain absorption spectrum line of target gas molecules (isotopes), and the absorption rate meets the Beer-Lambert relational expression. The database HITRAN discloses absorption line parameters for tens of major atmospheric molecules and their isotopes. During measurement, in order to avoid the influence of the effects of laser power drift, optical medium transmissivity fluctuation and the like, the wavelength is generally required to be scanned, the complete envelope of an isolated spectral line of target gas molecules is obtained by recording a laser spectrum in a certain waveband range, and the influence of a linear function is eliminated in an integral mode, so that the concentration of the target gas is obtained.
However, under normal temperature or high temperature conditions, even at low pressure, the doppler broadening (full width at half maximum) of molecular near-infrared transition reaches the level of several hundred MHz, and due to the presence of other molecules (or isotopes) in the background gas, if the content of target gas molecules (isotopes) is low or the absorption line is weak (i.e. when the target gas is a trace gas), the line is easily covered by the absorption of other background gases, and the absorption signal of the target line is buried in the background and cannot be extracted, thereby causing measurement failure.
Therefore, in the prior art, a mode of measuring a molecular saturation absorption spectrum of a trace gas is generally adopted, and it is expected that the concentration measurement of the trace gas in the gas to be measured is realized by utilizing the advantages that the molecular saturation absorption spectrum has a narrow line width and cannot be influenced by gas background gas absorption.
Disclosure of Invention
In order to solve the technical problem, the application provides a trace gas detection method and a trace gas detection system, so as to achieve the purpose of detecting the concentration of the trace gas or isotope to be detected by using the cavity of the trace gas or isotope to be detected to enhance the molecular saturated absorption spectrum without the doppler effect under the normal temperature condition.
In order to achieve the technical purpose, the embodiment of the application provides the following technical scheme:
a method of trace gas detection, comprising:
providing a resonant cavity, and filling gas to be detected in the cavity of the resonant cavity, wherein the gas to be detected comprises trace gas to be detected;
providing detection light rays with different frequencies, wherein the detection light rays are incident into the resonant cavity from one end of the resonant cavity in the extension direction and are emitted from the other end of the resonant cavity in the extension direction, so as to obtain the detection light rays carrying the information of the trace gas to be detected; the cavity of the resonant cavity has a telescopic degree of freedom in the extension direction of the resonant cavity so as to match the longitudinal mode frequency of the resonant cavity with the frequency of incident detection light;
according to detection light rays with different frequencies and carrying information of the trace gas to be detected, a molecular saturation absorption spectrum of the trace gas to be detected is obtained, and according to the molecular saturation absorption spectrum of the trace gas, the concentration of the trace gas to be detected in the gas to be detected is calculated.
Optionally, the method includes: the device comprises a laser generating device, a resonant cavity, a photoelectric detection device, a feedback control device and a scanning control device; wherein the content of the first and second substances,
the cavity of the resonant cavity is filled with gas to be detected, and the gas to be detected comprises trace gas to be detected; the cavity of the resonant cavity has a telescopic degree of freedom in the extending direction of the resonant cavity;
the laser generating device is used for providing detection light rays with different frequencies under the control of the feedback control device, wherein the detection light rays are incident into the resonant cavity from one end of the resonant cavity in the extension direction and are emitted from the other end of the resonant cavity in the extension direction so as to obtain the detection light rays carrying the information of the trace gas to be detected;
the photoelectric detection device is used for converting detection light carrying information of the trace gas to be detected into detection information in an electric signal form;
the scanning control device is used for recording the detection information in the form of the electric signal and controlling the laser generating device to adjust the frequency of the emergent detection light in a stepping mode;
and the feedback control device is used for controlling the frequency of the detection light emitted by the laser generating device to be matched with the longitudinal mode frequency of the resonant cavity.
Optionally, the resonant cavity includes:
the shell is provided with an incident end and an emergent end which are oppositely arranged;
the first reflector is positioned in the shell and close to one side of the incident end, and an antireflection film is arranged on one side, facing the incident end, of the first reflector;
the second reflector is positioned in the shell and close to one side of the emergent end, and reflecting surfaces of the first reflector and the second reflector are oppositely arranged;
a piezoelectric device disposed adjacent to the first mirror and/or adjacent to the second mirror; the piezoelectric device is used for pushing the first reflecting mirror and/or the second reflecting mirror adjacent to the piezoelectric device to move according to the received control electric signal, so that the cavity of the resonant cavity has a telescopic degree of freedom in the extending direction of the resonant cavity.
Optionally, the feedback control device includes: the system comprises a radio frequency signal source, a phase detection device and a PID amplification device; wherein the content of the first and second substances,
the radio frequency signal source is used for generating a sinusoidal radio frequency signal;
the phase detection device is used for converting detection information in the form of an electric signal into an error signal according to the sinusoidal radio frequency signal;
and the PID amplifying device is used for converting the error signal into a feedback locking signal so as to enable the frequency of the detection light emitted by the laser generating device to be matched with the longitudinal mode frequency of the resonant cavity.
Optionally, the laser generating device includes: a laser, a frequency modulation device and a coupling lens;
the laser is used for generating laser;
the frequency modulation device is used for modulating the laser generated by the laser according to the feedback locking information so as to obtain detection light with the frequency matched with the longitudinal mode frequency of the resonant cavity;
the coupling lens is used for coupling the detection light and then enabling the detection light to enter the resonant cavity.
Optionally, the frequency modulation device is an electro-optical modulator.
Optionally, the scan control device includes: information storage means and frequency scanning means; wherein the content of the first and second substances,
the information storage device is used for recording the detection information in the form of the electric signal;
and the frequency scanning device is used for generating a control electric signal and transmitting the control electric signal to the laser.
Optionally, the scanning control device is further configured to obtain a molecular saturation absorption spectrum of the to-be-detected trace gas according to the recorded detection information in the form of the electrical signal, and calculate the concentration of the to-be-detected trace gas in the to-be-detected gas according to the molecular saturation absorption spectrum of the trace gas.
Optionally, the photodetection device includes: the device comprises a lens matching module and a photoelectric detection module; wherein the content of the first and second substances,
the lens matching module is used for matching the spatial mode of the optical field in the photoelectric detection device;
and the photoelectric detection module is used for converting detection light carrying trace gas information to be detected into detection information in an electric signal form.
According to the technical scheme, the embodiment of the application provides a detection method and a detection system for trace gas, wherein the detection method for the trace gas fills gas to be detected in a resonant cavity, and utilizes the resonant cavity to enhance the laser power of detection light incident into the resonant cavity on one hand and improve the saturation parameter of molecular transition of the trace gas to be detected; on the other hand, the effective absorption range of the trace gas to be detected is enhanced, so that the detection sensitivity of weak absorption of the trace gas to be detected is improved, the molecular saturation absorption spectrum of the trace gas to be detected is obtained, and the purpose of detecting the concentration of the trace gas can be realized by utilizing the detection light provided by a conventional laser under the normal temperature condition.
In addition, due to the Doppler spread elimination characteristic of the molecular saturated absorption spectrum, the obtained molecular saturated absorption spectrum of the to-be-detected trace gas and the absorption spectrum of background gas molecules in the to-be-detected gas are effectively separated, so that the interference of the background gas is eliminated, and the detection of the to-be-detected trace gas is realized. The trace gas detection method provided by the embodiment of the application is particularly effective for detecting molecules of specific isotopes because the saturated absorption spectrum frequencies of different isotopes of the molecules are obviously different.
Drawings
In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the drawings needed to be used in the description of the embodiments or the prior art will be briefly introduced below, it is obvious that the drawings in the following description are only embodiments of the present application, and for those skilled in the art, other drawings can be obtained according to the provided drawings without creative efforts.
FIG. 1 is a schematic flow chart of a method for detecting trace gases according to an embodiment of the present disclosure;
FIG. 2 is a schematic diagram of a resonant cavity according to an embodiment of the present application;
FIG. 3 is a schematic diagram of a trace gas detection system according to an embodiment of the present application;
FIG. 4 shows a trace amount of gas to be measured in the simulated gas to be measured (A)12C16O) partial pressure and its R (19) transition lamb dip area;
fig. 5 is a molecular saturation absorption spectrum of the measured cavity-enhanced molecular V-3-0, R (19) transition.
Detailed Description
As mentioned in the background, the currently used laser absorption spectroscopy technique is to measure the absorption rate of a certain absorption line of target gas molecules (isotopes), and it satisfies the Beer-Lambert relation:
Tv＝Iv/I0＝exp(-kvL)；
wherein, TvRepresenting the transmission of the laser light through the L path in the medium, kvFor absorption, it is the concentration X of the target gas molecules measured under pressure PgasAbsorption line intensity S (T) and linear function
And a linear function satisfies
Where v represents frequency. Absorption rate kvCan be expressed as:
wherein the content of the first and second substances,
is a linear function obtained by convolution of a Gaussian Doppler broadening line and a Lorentz pressure broadening line, and the linear function satisfies
The database HITRAN discloses absorption line parameters (including line centers, s (t), etc.) for tens of major atmospheric molecules and their isotopes. During measurement, in order to avoid the influence of laser power drift, fluctuation of optical medium transmissivity and other effects, the wavelength is usually scanned, a complete envelope of an isolated spectral line of a target gas molecule is obtained by recording a laser spectrum in a certain waveband range, and the complete envelope is eliminated in an integral mode
The influence of the linear function, and thus the concentration of the target gas.
However, under normal or high temperature conditions, even at low pressure, the doppler broadening (full width at half maximum) of molecular near-infrared transitions reaches the level of several hundred MHz, and due to the presence of other molecules (or isotopes) in the background gas, if the content of target gas molecules (isotopes) is low or the absorption lines are weak, the lines are easily covered by the absorption of other background gases, and the absorption signals of the target lines are buried in the background and cannot be extracted, thereby causing measurement failure.
The molecular saturation absorption spectrum is that when a narrow line width exciting light excites partial molecules to an upper state, so that the number of the molecules which are arranged in a lower state is reduced, and an absorption signal measured by a detection light is reduced. If the excitation and detection light frequencies are the same and opposite, then an absorption cavity without doppler broadening and with a much narrower line width, called a lamb cavity, will be obtained since both simultaneously interact only with molecules with zero lateral velocity. The molecular saturation absorption spectrum line width is generally narrower than the Doppler spread width by about three orders of magnitude, so that the influence of other background gas absorption is avoided, and the detection selectivity can be greatly improved.
The depth Δ α of the molecular saturation absorption peak (lamb dip) can be represented by the following formula:
wherein, Piα for the partial pressure of the gas to be measuredmP is the absorption coefficient of the molecule to be detected without considering the saturation effect, S is the saturation parameter and can be calculated by the following formula:
wherein, IsTo saturation power, Is0Is the saturation power of the gas to be measured at the zero pressure limit,Pthe pressure broadening coefficient is used for describing the relationship between the line width of the molecular saturated absorption spectrum and the gas pressure,Tfor the transit time broadening, P is the total pressure of the sample gas. And the area of the lamb depressions can be obtained by the following formula:
whereinFWHMIs the full width at half maximum of the lamb dip. And calculating the concentration of the gas to be measured according to the measured area of the lamb pits.
However, there are many difficulties in detecting trace molecules using molecular saturation absorption spectroscopy. Firstly, the near-infrared oscillation transition moment of the molecule is very small, and under the condition of normal temperature, the transition broadening (about hundreds of kHz) of the molecule is far greater than the natural broadening (sub Hz or even smaller), so that the transition saturation can be caused only by needing very high laser power (more than kW/cm 2). Continuous wave semiconductor lasers commonly used in gas sensing do not meet the required requirements. Meanwhile, the saturated absorption is significantly affected by the pressure broadening, and in order to obtain a saturated absorption spectrum with sufficient contrast, measurement at low pressure (10Pa or less) is generally required, which puts higher requirements on measurement sensitivity.
In view of this, the present application provides a method for detecting a trace gas, including:
providing a resonant cavity, and filling gas to be detected in the cavity of the resonant cavity, wherein the gas to be detected comprises trace gas to be detected;
providing detection light rays with different frequencies, wherein the detection light rays are incident into the resonant cavity from one end of the resonant cavity in the extension direction and are emitted from the other end of the resonant cavity in the extension direction, so as to obtain the detection light rays carrying the information of the trace gas to be detected; the cavity of the resonant cavity has a telescopic degree of freedom in the extension direction of the resonant cavity so as to match the longitudinal mode frequency of the resonant cavity with the frequency of incident detection light;
according to detection light rays with different frequencies and carrying information of the trace gas to be detected, a molecular saturation absorption spectrum of the trace gas to be detected is obtained, and according to the molecular saturation absorption spectrum of the trace gas, the concentration of the trace gas to be detected in the gas to be detected is calculated.
The detection method of the trace gas fills the gas to be detected in the resonant cavity, and utilizes the resonant cavity to enhance the laser power of detection light entering the resonant cavity on one hand and improve the saturation parameter of the molecular transition of the trace gas to be detected; on the other hand, the effective absorption range of the trace gas to be detected is enhanced, so that the detection sensitivity of weak absorption of the trace gas to be detected is improved, the molecular saturation absorption spectrum of the trace gas to be detected is obtained, and the purpose of detecting the concentration of the trace gas can be realized by utilizing the detection light provided by a conventional laser under the normal temperature condition.
In addition, due to the Doppler spread elimination characteristic of the molecular saturated absorption spectrum, the obtained molecular saturated absorption spectrum of the to-be-detected trace gas and the absorption spectrum of background gas molecules in the to-be-detected gas are effectively separated, so that the interference of the background gas is eliminated, and the detection of the to-be-detected trace gas is realized. The trace gas detection method provided by the embodiment of the application is particularly effective for detecting molecules of specific isotopes because the saturated absorption spectrum frequencies of different isotopes of the molecules are obviously different.
The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application, and it is obvious that the described embodiments are only a part of the embodiments of the present application, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present application.
The embodiment of the application provides a method for detecting trace gas, as shown in fig. 1, including:
s101: providing a resonant cavity, and filling gas to be detected in the cavity of the resonant cavity, wherein the gas to be detected comprises trace gas to be detected;
s102: providing detection light rays with different frequencies, wherein the detection light rays are incident into the resonant cavity from one end of the resonant cavity in the extension direction and are emitted from the other end of the resonant cavity in the extension direction, so as to obtain the detection light rays carrying the information of the trace gas to be detected; the cavity of the resonant cavity has a telescopic degree of freedom in the extension direction of the resonant cavity so as to match the longitudinal mode frequency of the resonant cavity with the frequency of incident detection light;
s103: according to detection light rays with different frequencies and carrying information of the trace gas to be detected, a molecular saturation absorption spectrum of the trace gas to be detected is obtained, and according to the molecular saturation absorption spectrum of the trace gas, the concentration of the trace gas to be detected in the gas to be detected is calculated.
It should be noted that the trace gas refers to a gas component having an extremely low component content (on the order of ppm or less) in a gas sample at a certain pressure, and for example, carbon dioxide or methane in the atmosphere and its isotope gas, etc. can be considered as trace gas.
One embodiment of the present application provides a possible structure of a resonant cavity, which in this embodiment comprises a housing, a first mirror 22, a second mirror 23, and at least one piezoelectric device (not shown in fig. 2) arranged in the housing 21, with reference to fig. 2; the housing 21 comprises an incident end and an emergent end which are arranged oppositely, the first reflector 22 and the second reflector 23 are respectively arranged close to the incident end and the emergent end, and the reflecting surfaces of the first reflector 22 and the second reflector 23 are opposite; an antireflection film may be attached to the first reflector 22 near the incident end toward the incident end to increase the transmittance of the incident detection light.
In this embodiment, the method for detecting a trace gas fills a gas to be detected in a resonant cavity, and the resonant cavity is utilized to enhance the laser power of a detection light beam incident into the resonant cavity on one hand and to improve the saturation parameter of the molecular transition of the trace gas to be detected; on the other hand, the effective absorption range of the trace gas to be detected is enhanced, so that the detection sensitivity of weak absorption of the trace gas to be detected is improved, the molecular saturation absorption spectrum of the trace gas to be detected is obtained, and the purpose of detecting the concentration of the trace gas can be realized by utilizing the detection light provided by a conventional laser under the normal temperature condition.
In addition, due to the Doppler spread elimination characteristic of the molecular saturated absorption spectrum, the obtained molecular saturated absorption spectrum of the to-be-detected trace gas and the absorption spectrum of background gas molecules in the to-be-detected gas are effectively separated, so that the interference of the background gas is eliminated, and the detection of the to-be-detected trace gas is realized. The trace gas detection method provided by the embodiment of the application is particularly effective for detecting molecules of specific isotopes because the saturated absorption spectrum frequencies of different isotopes of the molecules are obviously different.
The following embodiments of the present application provide a trace gas detection system capable of implementing a trace gas detection method, as shown in fig. 3, the trace gas detection system includes: the laser generating device 10, the resonant cavity 20, the photoelectric detection device 30, the feedback control device 40 and the scanning control device 50; wherein the content of the first and second substances,
the cavity of the resonant cavity 20 is filled with a gas to be detected, and the gas to be detected comprises a trace gas to be detected; the cavity of the resonant cavity 20 has a degree of freedom of expansion and contraction in the direction of extension of the resonant cavity 20;
the laser generating device 10 is configured to provide detection light rays with different frequencies under the control of the feedback control device 40, where the detection light rays enter the resonant cavity 20 from one end of the resonant cavity 20 in the extending direction and exit from the other end of the resonant cavity 20 in the extending direction, so as to obtain detection light rays carrying information of a trace gas to be detected;
the photoelectric detection device 30 is used for converting detection light carrying information of the trace gas to be detected into detection information in the form of an electric signal;
the scanning control device 50 is used for recording the detection information in the form of the electric signal and controlling the laser generating device 10 to adjust the frequency of the emitted detection light in a stepping mode;
the feedback control device 40 is configured to control the frequency of the detection light emitted by the laser generator 10 to match the longitudinal mode frequency of the resonant cavity 20.
In this embodiment, since the feedback control device 40 can control the frequency of the detection light emitted from the laser generator 10 to match the longitudinal mode frequency of the resonant cavity 20, the scan control device 50 can only be used to control the laser generator 10 to adjust the frequency of the emitted detection light in a step-by-step manner, and when the frequency of the detection light emitted from the laser generator 10 is changed under the control of the scan control device 50, the feedback control device 40 adjusts the longitudinal mode frequency of the resonant cavity 20 and/or the frequency of the detection light emitted from the laser generator 10 in real time, so that the frequency of the detection light emitted from the laser generator 10 matches the longitudinal mode frequency of the resonant cavity 20.
In addition, the purpose of controlling the laser generating device 10 to adjust the frequency of the emitted detection light in a stepping manner by the scanning control device 50 is to enable the scanning control device 50 to record detection information in an electrical signal form corresponding to the information of the trace gas to be detected carried by the detection light with different frequencies, so as to obtain a molecular saturation absorption spectrum of the trace gas to be detected according to the recorded detection information in the electrical signal form, and calculate the concentration of the trace gas to be detected in the gas to be detected according to the molecular saturation absorption spectrum of the trace gas.
Optionally, in an embodiment of the present application, the scanning control device 50 is further configured to obtain a molecular saturation absorption spectrum of the to-be-detected trace gas according to the recorded detection information in the form of the electrical signal, and calculate the concentration of the to-be-detected trace gas in the to-be-detected gas according to the molecular saturation absorption spectrum of the trace gas.
Referring to fig. 2, the resonant cavity 20 includes:
a housing 21, the housing 21 having an incident end and an exit end oppositely disposed;
the first reflector 22 is positioned inside the shell 21 and close to one side of the incident end, and an antireflection film is arranged on one side, facing the incident end, of the first reflector 22;
a second reflector 23 positioned inside the housing 21 and close to one side of the emergent end, wherein the reflecting surfaces of the first reflector 22 and the second reflector 23 are oppositely arranged;
a piezoelectric device disposed adjacent to the first mirror 22 and/or adjacent to the second mirror 23; the piezoelectric device is used for pushing the first mirror 22 and/or the second mirror 23 adjacent to the piezoelectric device to move according to the received control electric signal, so that the cavity of the resonant cavity 20 has a telescopic degree of freedom in the extending direction of the resonant cavity 20.
In this embodiment, the first mirror 22 and the second mirror 23 form a cavity of the resonant cavity 20 therebetween, so that the detection laser can be cavity-enhanced in the cavity.
The antireflection film located on the side of the first reflector 22 facing the incident end is used to increase the transmittance of the detection light incident on the first reflector 22, and improve the light energy utilization rate of the detection light.
Optionally, the feedback control device 40 includes: a radio frequency signal source, a phase detection device and a PID (proportion-integration-differentiation) amplification device; wherein the content of the first and second substances,
the radio frequency signal source is used for generating a sinusoidal radio frequency signal;
the phase detection device is used for converting detection information in the form of an electric signal into an error signal according to the sinusoidal radio frequency signal;
the PID amplifying device is configured to convert the error signal into a feedback locking signal, so that the frequency of the detection light emitted from the laser generator 10 matches the longitudinal mode frequency of the resonant cavity 20.
In this embodiment, the radio frequency signal source in the feedback control device 40 is configured to provide a working reference signal of a phase detection device, the phase detection device converts detection information in the form of an electrical signal into an error signal, and the PID amplifying device converts the error signal into a feedback locking signal and transmits the feedback locking signal to the piezoelectric device, so that the piezoelectric device adjusts the cavity length of the resonant cavity 20 in the extending direction of the resonant cavity 20 according to the feedback locking signal, so as to adjust the longitudinal mode frequency of the resonant cavity 20, and match the longitudinal mode frequency with the frequency of the detection light emitted by the laser generating device 10.
Accordingly, the scanning control device 50 only needs to control the laser generating device 10 to adjust the frequency of the emitted detection light in a step-by-step manner, and the feedback control device 40 adjusts the cavity length of the resonant cavity 20 so that the longitudinal mode frequency of the resonant cavity 20 matches the frequency of the detection light emitted by the laser generating device 10.
It should be noted that the smaller the variation step of the frequency of the detection light of the scanning control device 50 controlling the laser generating device 10 is, the more accurate the concentration of the gas to be measured obtained by the final calculation is. The frequency variation range of the detection light of the laser generator 10 controlled by the scan controller 50 may be in the order of megahertz or several hundred megahertz. The specific value of the change step length of the detection light and the specific frequency change range of the detection light are not limited, and the method is specifically determined according to the actual situation.
Optionally, the laser generator 10 includes: a laser, a frequency modulation device and a coupling lens;
the laser is used for generating laser;
the frequency modulation device is configured to modulate laser light generated by the laser device according to the feedback locking information to obtain a detection light with a frequency matching a longitudinal mode frequency of the resonant cavity 20;
the coupling lens is configured to couple the detection light and then to make the detection light enter the resonant cavity 20.
The laser may be of the conventional laser variety semiconductor, fiber or solid state. The present application does not limit this, which is determined by the actual situation.
Optionally, the frequency modulation device is an electro-optical modulator.
Optionally, the scan control device 50 includes: information storage means and frequency scanning means; wherein the content of the first and second substances,
the information storage device is used for recording the detection information in the form of the electric signal;
and the frequency scanning device is used for generating a control electric signal and transmitting the control electric signal to the laser.
In this embodiment, the scanning control device 50 controls the frequency of the laser light generated by the laser to change in a certain step by the generated control electric signal.
Optionally, the photodetection device 30 includes: the device comprises a lens matching module and a photoelectric detection module; wherein the content of the first and second substances,
the lens matching module is used for matching the spatial mode of the optical field inside the photodetection device 30;
and the photoelectric detection module is used for converting detection light carrying trace gas information to be detected into detection information in an electric signal form.
The following is a description of specific embodiments of the detection system for trace gas provided in the embodiments of the present application.
In this embodiment, the trace gas to be measured is12C16O gas molecules, the purpose being to measure12C16Molecular saturation absorption spectrum of infrared vibration transition of O gas molecules and obtaining the spectrum according to the area of the spectrum peak12C16Gas partial pressure of O gas molecules.
Optionally, the laser is an external cavity semiconductor laser, and its outputThe output laser enters the resonant cavity 20 after being modulated by an electro-optical modulator and coupled by a coupling lens; the resonant cavity 20 is filled with a gas to be measured including12C16O gas, the total pressure of the gas to be measured is P, and the total pressure is measured by a pressure gauge connected with the resonant cavity 20; the resonant cavity 20 is provided with a first reflector 22 and a second reflector 23, the reflectivity of the first reflector 22 and the reflectivity of the second reflector 23 reach 99.995%, the first reflector 22 and the second reflector 23 form an optical resonant cavity 20, and the back surface of the first reflector 22 or the second reflector 23 is connected with a piezoelectric device which can drive the mirror to slightly move along the optical path direction (the extending direction of the resonant cavity 20).
The detection light which is emitted from the resonant cavity 20 and carries the information of the trace gas to be detected is converted into detection information in the form of an electric signal by the photoelectric detection device 30, the detection signal in the form of the electric signal is divided into two paths of signals after being filtered and amplified, one path of signal is sent into the feedback control device 40, and a feedback locking signal is generated after being demodulated by the feedback control device 40, so that the longitudinal mode frequency of the resonant cavity 20 is matched with the frequency of the detection light emitted by the laser generation device 10; the other signal is sent to the scanning control device 50 for recording, and the scanning control device 50 controls the laser generating device to perform step-by-step frequency scanning.
Through the scanning process of the detection laser, the molecular saturation absorption spectrum of the trace gas to be detected can be obtained, referring to fig. 4 and 5, fig. 4 is the trace gas to be detected in the gas to be detected obtained through simulation: (fig. 4)12C16O) partial pressure and its R (19) transition lamb depression area, plotted on the abscissa in FIG. 412C16Partial pressure of O gas (partial pressure) in pascal (Pa) and Area of Lamb depression (Area of Lamb Dip) on the ordinate in 10-9cm-1MHz; FIG. 5 is a measured molecular saturable absorption spectrum of the cavity enhanced molecular V-3-0, R (19) transition; the abscissa in FIG. 5 is the Relative Frequency (Relative Frequency) in megahertz (MHz); the ordinate is the absorption coefficient (AbsorptionCoefficient) in 10-9cm-1. From FIG. 4, trace gas (b) to be measured12C16O) partial pressure and R (19) transition lamb depression area have good linear corresponding relation in a large range, and are suitable for quantitative measurement. Fitting the molecular saturated absorption spectrum shown in FIG. 5 to obtain the peak height, peak width and peak area thereof, and determining the trace gas to be detected (12C16O) partial pressure in the gas to be measured.
In summary, the embodiment of the present application provides a detection method and a detection system for trace gas, where the detection method for trace gas fills a gas to be detected in a resonant cavity, and utilizes the resonant cavity to enhance the laser power of a detection light incident into the resonant cavity on the one hand, and to improve the saturation parameter of the molecular transition of the trace gas to be detected; on the other hand, the effective absorption range of the trace gas to be detected is enhanced, so that the detection sensitivity of weak absorption of the trace gas to be detected is improved, the molecular saturation absorption spectrum of the trace gas to be detected is obtained, and the purpose of detecting the concentration of the trace gas can be realized by utilizing the detection light provided by a conventional laser under the normal temperature condition.
In addition, due to the Doppler spread elimination characteristic of the molecular saturated absorption spectrum, the obtained molecular saturated absorption spectrum of the to-be-detected trace gas and the absorption spectrum of background gas molecules in the to-be-detected gas are effectively separated, so that the interference of the background gas is eliminated, and the detection of the to-be-detected trace gas is realized. The trace gas detection method provided by the embodiment of the application is particularly effective for detecting molecules of specific isotopes because the saturated absorption spectrum frequencies of different isotopes of the molecules are obviously different.
The embodiments in the present description are described in a progressive manner, each embodiment focuses on differences from other embodiments, and the same and similar parts among the embodiments are referred to each other.
The previous description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the present application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the application. Thus, the present application is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.
Claims (9)
1. A method of detecting trace gas, comprising:
providing a resonant cavity, and filling gas to be detected in the cavity of the resonant cavity, wherein the gas to be detected comprises trace gas to be detected;
providing detection light rays with different frequencies, wherein the detection light rays are incident into the resonant cavity from one end of the resonant cavity in the extension direction and are emitted from the other end of the resonant cavity in the extension direction, so as to obtain the detection light rays carrying the information of the trace gas to be detected; the cavity of the resonant cavity has a telescopic degree of freedom in the extension direction of the resonant cavity so as to match the longitudinal mode frequency of the resonant cavity with the frequency of incident detection light;
according to detection light rays with different frequencies and carrying information of the trace gas to be detected, a molecular saturation absorption spectrum of the trace gas to be detected is obtained, and according to the molecular saturation absorption spectrum of the trace gas, the concentration of the trace gas to be detected in the gas to be detected is calculated.
2. A trace gas detection system, comprising: the device comprises a laser generating device, a resonant cavity, a photoelectric detection device, a feedback control device and a scanning control device; wherein the content of the first and second substances,
the cavity of the resonant cavity is filled with gas to be detected, and the gas to be detected comprises trace gas to be detected; the cavity of the resonant cavity has a telescopic degree of freedom in the extending direction of the resonant cavity;
the laser generating device is used for providing detection light rays with different frequencies under the control of the feedback control device, wherein the detection light rays are incident into the resonant cavity from one end of the resonant cavity in the extension direction and are emitted from the other end of the resonant cavity in the extension direction so as to obtain the detection light rays carrying the information of the trace gas to be detected;
the photoelectric detection device is used for converting detection light carrying information of the trace gas to be detected into detection information in an electric signal form;
the scanning control device is used for recording the detection information in the form of the electric signal and controlling the laser generating device to adjust the frequency of the emergent detection light in a stepping mode;
and the feedback control device is used for controlling the frequency of the detection light emitted by the laser generating device to be matched with the longitudinal mode frequency of the resonant cavity.
3. The system of claim 2, wherein the resonant cavity comprises:
the shell is provided with an incident end and an emergent end which are oppositely arranged;
the first reflector is positioned in the shell and close to one side of the incident end, and an antireflection film is arranged on one side, facing the incident end, of the first reflector;
the second reflector is positioned in the shell and close to one side of the emergent end, and reflecting surfaces of the first reflector and the second reflector are oppositely arranged;
a piezoelectric device disposed adjacent to the first mirror and/or adjacent to the second mirror; the piezoelectric device is used for pushing the first reflecting mirror and/or the second reflecting mirror adjacent to the piezoelectric device to move according to the received control electric signal, so that the cavity of the resonant cavity has a telescopic degree of freedom in the extending direction of the resonant cavity.
4. The system of claim 3, wherein the feedback control device comprises: the system comprises a radio frequency signal source, a phase detection device and a PID amplification device; wherein the content of the first and second substances,
the radio frequency signal source is used for generating a sinusoidal radio frequency signal;
the phase detection device is used for converting detection information in the form of an electric signal into an error signal according to the sinusoidal radio frequency signal;
and the PID amplifying device is used for converting the error signal into a feedback locking signal so as to enable the frequency of the detection light emitted by the laser generating device to be matched with the longitudinal mode frequency of the resonant cavity.
5. The system of claim 4, wherein the laser generating device comprises: a laser, a frequency modulation device and a coupling lens;
the laser is used for generating laser;
the frequency modulation device is used for modulating the laser generated by the laser according to the feedback locking information so as to obtain detection light with the frequency matched with the longitudinal mode frequency of the resonant cavity;
the coupling lens is used for coupling the detection light and then enabling the detection light to enter the resonant cavity.
6. The system of claim 5, wherein the frequency modulation device is an electro-optic modulator.
7. The system of claim 3, wherein the scan control means comprises: information storage means and frequency scanning means; wherein the content of the first and second substances,
the information storage device is used for recording the detection information in the form of the electric signal;
and the frequency scanning device is used for generating a control electric signal and transmitting the control electric signal to the laser.
8. The system according to claim 2, wherein the scanning control device is further configured to obtain a molecular saturation absorption spectrum of the trace gas to be detected according to the recorded detection information in the form of the electrical signal, and calculate the concentration of the trace gas to be detected in the gas to be detected according to the molecular saturation absorption spectrum of the trace gas.
9. The system of claim 2, wherein the photodetection device comprises: the device comprises a lens matching module and a photoelectric detection module; wherein the content of the first and second substances,
the lens matching module is used for matching the spatial mode of the optical field in the photoelectric detection device;
and the photoelectric detection module is used for converting detection light carrying trace gas information to be detected into detection information in an electric signal form.
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| CN201910456370.9A CN110160989B (en) | 2019-05-29 | 2019-05-29 | Trace gas detection method and detection device |
| US17/614,763 US20220228977A1 (en) | 2019-05-29 | 2020-03-30 | Detection method and detection device for trace gas |
| PCT/CN2020/082019 WO2020238386A1 (en) | 2019-05-29 | 2020-03-30 | Detection method and detection device for trace gas |
| EP20813712.5A EP3978906A4 (en) | 2019-05-29 | 2020-03-30 | Detection method and detection device for trace gas |
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| CN110160989B (en) * | 2019-05-29 | 2020-08-28 | 中国科学技术大学 | Trace gas detection method and detection device |
| CN112179624A (en) * | 2020-09-22 | 2021-01-05 | 北京航空航天大学 | Method and system for measuring eigenfrequency of external cavity semiconductor laser by using FP (Fabry-Perot) cavity power spectrum |
| CN112683846B (en) * | 2021-01-05 | 2022-10-28 | 中国科学技术大学 | Trace gas detection device and method |
| CN114076747B (en) * | 2021-11-23 | 2023-12-19 | 北京交通大学 | Multi-gas detection device and method based on lock mold cavity enhanced absorption spectrum |
| CN114486808B (en) * | 2022-01-12 | 2023-07-04 | 山东大学 | Gas detection method for enhancing spectral line absorption intensity |
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